The present invention relates to a laser lap welding method for parts made of galvanized steel sheet, or more specifically, to a laser lap welding method suitable as an alternative technique to be carried out in place of spot welding.
Galvanized sheets are used in many portions of vehicle bodies of automobiles in consideration of corrosion resistance. Panels made of galvanized sheets to form vehicle body parts are press-formed into three-dimensional shapes and are welded together at peripheral portions of such three-dimensional shapes, and thereby form a vehicle body structure based on a hollow cross-section which has an advantage in strength. Generally, such vehicle body panels have been integrated mainly by means of spot welding. However, laser welding is now being introduced as an alternative technique that allows processing at a higher speed.
It is known that vaporized zinc may cause welding failures such as blowholes when galvanized sheets are closely overlapped and welded to each other using a laser, as the vaporized zinc may blow fused metal away or may remain in the fused metal as bubbles. To avoid this problem, JP10-216974A and JP2571976B disclose a technique in which protrusions are formed on any one of galvanized sheets to form a gap for discharging the zinc vapor in a state in which the sheets are overlapped on each other.
Since a vehicle body panel of an automobile is formed by press forming, protrusions are formed on such pressed parts by subjecting the parts to an embossing process with punches arranged inside a press die after a press forming process. However, in the case of employing laser welding for parts originally designed to be suitable for spot-welding, the following problems occur because such parts are not designed to allow arrangement of the protrusions necessary for laser welding. For example, the protrusions cannot be disposed due to limited space and presence of inclinations on a joining surface, or welding positions need to be changed because of the protrusions added.
If the welding positions are changed, a strength performance and an impact resistance performance as a vehicle body structure are changed. For this reason, the structure will need to undergo performance confirmation tests again. Moreover, the parts that are newly designed cannot directly utilize design data which have been accumulated on the premise of spot welding. Such problems have been considerable obstacles to introduction of laser welding.